For the most part, it's ease of set up. The rack gets set down in the same configuration and with all the tom and cymbal arms already set, you just plunk everything down and you're set up the same every time.
With stands (even with memory locks) you have to make sure the feet are in the right spot and fiddle a lot more to get your set up the same.
Another benefit is stability.
Spend a little time on the Internet looking at DW/PDPs, Gibraltar, Yamaha, Pearl racks. It will all make sense. Some have straight tubes, curved, V shaped.